A system in combination of a photovoltaic power generation device (a solar cell), a light-emitting member, and a secondary cell can be used as a stand-alone lighting device with no power source. For example, a lighting device in combination of a silicone solar cell, a white LED (a light-emitting member), and a secondary cell has been proposed and commercialized. However, since the above white LED is a point light source, and an electric power generation layer of the above silicone solar cell is made of a crystal, the above lighting device is unable to be provided with flexibility, and thereby, it was not made to be a compact independent lighting device such that it can be stored in a roll form or carried with the device folded.
On the other hand, both an organic electroluminescence element and an organic photoelectric conversion element are an electronic device which is all solid state and in a form of a thin film. Due to the above reasons, it has been expected that, by using the above organic electroluminescence element as the light-emitting member and the above organic photoelectric conversion element as the solar cell, and by combining them with the solar cell, there can be constituted a lighting device which needs no power source and is light, thin, flexible, and portable. It can be assumed that such a lighting device can not only be used for an outdoor leisure use but also contribute to the spread of lighting in developing countries where development of infrastructure such as electricity is delayed.
However, the organic electroluminescence element and organic photoelectric conversion element have a problem of low durability, and there is a need for improvement.
In addition, in the above lighting device in combination of a light-emitting member, a solar cell, and a secondary cell, a constitution and a driving method of the lighting device have so far been disclosed, in which prevention of degradation of the secondary cell was focused.
For example, it has been disclosed that, in response to the degradation of the above secondary cell, in a lighting device constituted of a light-emitting element (a light emitting member), a solar cell, and a secondary cell, an over discharge control circuit detects electric energy stored in the secondary cell, and then, based on the detection signal, controls power supply from the secondary cell (for example, refer to Patent Document 1).
In addition, it has been disclosed that, in the above lighting device, open voltage of the secondary cell is measured, and then, charging is controlled by comparing the measured open voltage with prescribed overcharge protection voltage (for example, refer to Patent Document 2).
However, these lighting devices are constituted of an inorganic LED which is hard to be bent, a crystal silicon solar cell which is likely to break when dropped, a very heavy lead-acid storage battery, and the like, and therefore, the use, in which a light device which is unlikely to break is assumed to be carried around, is not taken into consideration, and further, there are no descriptions on problems in the case where a light and thin light-emitting element (an organic electroluminescence element) suitable for carrying around, photoelectric conversion element (organic photoelectric conversion element), a secondary cell (a lithium-ion battery), and the like are combined.